The initial components of the visual system of the giant barnacle include four extraordinarily large photoreceptors whose somata, axons, and terminal regions are accessible for intracellular recording, and postsynaptic cells which also permit such recording. The similarity of certain anatomical and physiological features of this visual system to those of the vertebrate retina make it of particular interest for studying photoreceptor membrane and synaptic properties as well as early visual integration. The regional membrane properties of the receptor will be studied by isolating portions of the cell from one another; in particular, the unusually high membrane resistivity of the axon and the regenerative properties of the terminals will be investigated with current- and voltage-clamp methods and with freeze-fracture electron microscopy. Simultaneous intracellular recordings from a receptor terminal region and from a second-order cell will establish the input-output relationship of the synapse, the timecourse and voltage dependence of transmitter release, and the nature of possible feedback interactions. Integration of visual information with that from other sensory systems will be studied by recording from higher-order neurons.